Sensors are used to determine the adjustment of a fuel-air mixture for operation of an internal combustion engine by determining the oxygen concentration in the exhaust gas of the internal combustion engine. The fuel-air mixture may be in the rich range, i.e., the fuel is present in stoichiometric excess, so that only a small quantity of oxygen is present in the exhaust gas in comparison with other partially unburned components. In the lean range, where there is more oxygen than air in the fuel-air mixture, the oxygen concentration in the exhaust gas is high accordingly.
Lambda probes are known for determining the oxygen concentration in the exhaust gas; these probes detect a lambda value of &gt;1 in the lean range or &lt;1 in the rich range, and a lambda value=1 in the stoichiometric range. In a known way, a Nernst measurement cell of the sensor supplies a detection voltage which is sent to a circuit arrangement. With the help of this circuit arrangement, the detection voltage is transformed to a pump voltage for a measurement probe (pump cell) which is also a part of the sensor. The measurement probe functions as a pump cell, where oxygen ions are pumped from a first electrode to a second electrode of the pump cell or vice versa, depending on the prevailing oxygen concentration in the gas mixture on which the measurement is to be performed. Depending on whether the lambda probe detects a rich range, i.e., a lambda value&lt;1, or a lean range, i.e., a lambda value&gt;1, the circuit arrangement determines whether an electrode of the pump cell connected to an active input of the circuit arrangement is switched as an anode or a cathode. The second electrode of the pump cell is connected to ground, so that either a cathodic limit current is set at the pump cell with a rich measurement gas or an anodic limit current is set with a lean measurement gas.
With a known sensor design, an electrode of the Nernst measurement cell and an electrode of the pump cell are each arranged in a joint cavity of the sensor, which is exposed to the exhaust gas through a diffusion barrier. If the fuel-air mixture to be monitored is in the lean range for a long period of time, oxygen ions diffuse out of the exhaust gas through the diffusion barrier into the joint cavity of the Nernst electrode of the Nernst measurement cell and the one pump electrode of the pump cell. According to the higher oxygen content in the lean range, an anodic limit current is applied to the pump cell by the circuit arrangement. In this way, additional oxygen ions are pumped into the joint cavity through the pump cell. One disadvantage of this is that if the internal combustion engine operates under lean conditions for a long period of time, e.g., several hours, fewer oxygen ions are pumped into the joint cavity of the Nemst electrode and the one pump electrode through the pump cell than would be necessary to maintain .lambda.=1 in the cavity. This is due to falsification of the voltage of the Nernst measurement cell due to the participation of the Nernst electrode in the function of the internal pump electrode. This is the case when the internal pump electrode has become inactive due to long-lasting cathodic operation or due to manufacturing tolerance. However, due to the increasing concentration of oxygen ions in the joint cavity, the Nernst measurement cell determines that the fuel-air mixture is becoming richer, so that the sensor is subject to a rich drift leading to inaccuracies in the output signal.